The development of high energy cell systems requires the compatibility of an electrolyte possessing desirable electrochemical properties with highly active anode materials, such as lithium, calcium, sodium and the like, and the efficient use of high energy density cathode materials, such as FeS.sub.2, Co.sub.3 O.sub.4, PbO.sub.2, TiS.sub.2, Li.sub.x CoO.sub.2, MnO.sub.2, (CF.sub.x).sub.n, oxyhalides and the like. One drawback in the manufacturing and assembling of these high energy cell systems is that the anodes, such as lithium, are usually soft, sticky material having a tendency to stick together when they are transported between areas for assembly into cells. In addition, anodes such as lithium have a tendency to accumulate static charge buildup during their transportation from the manufacturing facilities to the assembling facilities.
To achieve optimum battery performance with respect to power output, the surface of electrodes, such as a lithium electrode, should be made as large as is practicable. To achieve an electrode having an optimum surface area, it is generally necessary to roll the lithium metal to a predetermined length and thickness. When attempts have been made to roll and shape sheet or foil of lithium, the lithium may adhere to the rollers and as a result, efforts at rolling thin lithium sheets or foils have generally been extremely difficult.
In the manufacture of soft, sticky strips of lithium, it has been proposed in the prior art, specifically U.S. Pat. No. 3,721,113, that thin continuous lithium strips can be produced by cold rolling lithium metal while it is compressed between smooth surfaces of a solid polymeric composition, which composition is nonreactive with lithium and has a critical surface tension of not over 46 dynes per centimeter at 20.degree. C. The use of the polymeric sheet material is essential so as to prevent the sticking of the lithium to the metal surfaces of the roller. Once continuous lithium strips are produced, another problem encountered is in the cutting of the lithium metal into a plurality of pieces which can be employed as lithium anodes in various types of cell systems. To overcome this problem it is disclosed in the prior art, specifically U.S. Pat. No. 4,060,017, that a flexible film, preferably of plastic, be interposed between the blade of a cutting device and a lithium strip such that when the blade is forced against the anvil with sufficient force to cut the lithium, the film prevents contact of the blade with the lithium. This will prevent any buildup of lithium being developed on the blade which would occur generally after only a few cutting operations. Although solutions in the prior art have been proposed for the manufacture of a plurality of lithium electrodes from lithium strip material, one problem that still exists is in the transporting of the lithium electrodes from the manufacturing site to the assembly site where they will be assembled into a cell. Rectangular, square, circular or other shaped lithium electrodes are usually transported to the assembly station where they are then fed into automatic feeding machines and assembled into a cell container. It has been observed that during the transporting to and automatic feeding of the lithium anodes at the assembly station, there is a tendency for freshly cut lithium anodes to stick to themselves and to accumulate static charge whereupon the anodes then tend to stick to other surfaces resulting in a disruption of the assembly operation. This problem is most pronounced in employing lithium anodes for miniature type button cells in which the anodes can be as small as a square measuring 0.22 inch by 0.10 inch thick. An additional Problem encountered during the transporting to and the feeding of lithium anodes at the assembly site is that there is a tendency for a coating of the lithium to build up on any metallic or plastic surface that they contact. In the manufacture of lithium strips, this tendency of the lithium buildup on metal surfaces can be eliminated through the use of polymeric materials as described above in conjunction with U.S. Pat. No. 3,721,113 and in the use of a flexible plastic film as discussed above in conjunction with U.S. Pat. No. 4,060,017. These solutions cannot be efficiently employed in the transportation of small size lithium anodes to the assembly site since such anodes are relatively small discrete bodies and any attempts to apply a plastic or polymeric material between the anodes would be time consuming, laborious and expensive.
U.S. Pat. No. 4,315,976 discloses a soft active anode coated on its surface with an electrically nonconductive and chemically and electrochemically inert particulate material, such as talc, in an amount between about 0.1 milligram per square centimeter and about 8 milligrams per square centimeter, preferably between about 0.3 milligram per square centimeter and about 4 milligrams per square centimeter, so as to retard static buildup and the tendency of the anode to stick to surfaces and other anodes prior to its assembly into a cell.
In addition to means for facilitating the handling of soft electrodes, a separator is generally disposed against the electrode in an assembled cell. Conventional type nonwoven glass fiber separators employing organic binders such as polyvinyl alcohol (PVA) have created problems in nonaqueous cells, such as lithium thionyl chloride cells. Specifically, the PVA binder has been found to cause gassing in the cell which results in premature venting on high temperature storage. To alleviate this problem, separators with a very low binder content or ones containing no binder could be used. Japanese application 1975/64735 discloses the use of glass beads on one side of an AgCl cathode as a separator in an Mg/AgCl seawater battery. Japanese Patent Application 1982/172660 discloses the use of glass beads embedded within a lithium anode to serve as a separator for a lithium cell. The glass beads are used to provide a space between the lithium anode and a positive electrode that will permit circulation of the cell's electrolyte solution between the electrodes of the cell.
U.S. Pat. No. 4,158,085 discloses an electrode with separator beads embedded therein, such beads having a coating of adhesive to improve their adherence to the electrode surface.
One of the objects of the present invention is to provide an electrode-separator composite for an electrochemical cell which comprises a microporous separator composed of a layer of insulating microspheres partially embedded in the surface of the electrode, thereby producing an integral electrode and separator composite that is easy to handle prior to and during assembly operations and a composite that will occupy a relatively small space in an electrochemical cell.
Another object of the present invention is to provide a binder-free separator composed of microspheres that are partially embedded in the surface of an electrode to produce a microporous separator having a micronodular surface contour and secured in the surface of a soft electrode.
Another object of the present invention is to partially embed a layer of glass or ceramic microspheres into the surface of a soft electrode and wherein said microspheres have a diameter between about 5 microns and about 0.05 inch.